BMA Viscount Spool-up Behaviour
Join Date: Feb 2006
Location: UK
Posts: 592
Likes: 0
Received 0 Likes
on
0 Posts
Pink Floyd - Dark Side of The moon - On The Run, about 3 minutes in, that has just got to be a Viscount running up for takeoff 
Thank you for adding second and third opinions to what I have thought ever since the album was released! Sad really!
Thank you for adding second and third opinions to what I have thought ever since the album was released! Sad really!
I spent many happy days as a kid at the western end of 10R watching streams of BEA and other( LH KL EI ....) Viscounts whistle their cheeerful way to the Block 79?? holding point before lining up and bingo just as described here a clearly perceptible pause and drop in sound level as the whistle disappeared and was repalced by a deeper growl as the brakes came off and they disappeared towards the east. Very very distinctive and fascinating to read this thread describing possible reasons for it.
I only flew on the Viscount a couple of times-to Jersey I think but a really nice airplane from a pax view point and those windows wow!
PB
I only flew on the Viscount a couple of times-to Jersey I think but a really nice airplane from a pax view point and those windows wow!
PB
Delete references to "core spool" and "2-spool"!
Quote from jh5speed:
Just to note the Dart is a single spool animal - prop is connected to the one (and only) shaft. 2 stage of compressor (centrifugal) and 2 turbine stages though - perhaps that's what you were thinking of?
[Unquote]
Yes... Mea culpa, I've belatedly looked at a cut-away. Shouldn't have relied on memory, 37 years on. You, ABUKABOY and barit1 didn't need any help from me, you were spot on.
I think barit1 is right, a slower advance of the throttles avoids the momentary drop in rpm as the prop comes out of ground-fine into governing mode. On the Herald, the latter was announced by the pitch lever moving forward (provided the controls had been unlocked), and the green ground-fine-pitch lights going out.
For the landing, the Viscount selects its own ground-fine, apparently, by the weight switches. On the Herald, the crew had to remember it. Don't know about the F27.
411A is right that there is a change of note if/when water-meth cuts in. The note gets heavier (more thrust from coarser pitch at the same maximum rpm?) and, on the Herald, heard externally, there was often a strange, slapping sound.
Quote from dixi188:
Also the -520 series Dart, on Viscounts and F27s, W/M restores power at hight ambient temps so would not be used in the UK very often except on hot summer days, where as the -532 on the Dart Herald boosted power at all temps. so was used more often.
[Unquote]
That would explain why on the Herald (only 2 engines, if my memory hasn't failed again)
we often used it, together with zero flap, to satisfy the Performance-A "WAT" limit (to improve the second-segment climb angle in the engine-out case); even with fairly modest summer temperatures.
[Rather off-topic, "Whispering Giant" was a marketing slogan for the (Proteus) Britannia. Certainly quiet inside, from my (pax) experience. But, as you say, there was a deep rumble in the air, heard externally. The Bristol Proteus was 2-spool (I checked this time!), with axial-flow compressors on the N2, and 2 turbines per spool; but the air flowed forwards while being compressed, then reversing direction into the combustion chambers. This enabled the engine to be shorter (but caused the serious African icing problems early on). When taxying, at idle revs, it made an almost inaudible whisper (observed from the perimeter fence), occasionally interrupted by a "whoosh" - maybe as the props went into superfine pitch when the throttles were completely closed?
So unlike the deafening high-pitched song of the Dart... I would love to have a recording of the Proteus, but doubt finding one on a pop album.]
Just to note the Dart is a single spool animal - prop is connected to the one (and only) shaft. 2 stage of compressor (centrifugal) and 2 turbine stages though - perhaps that's what you were thinking of?
[Unquote]
Yes... Mea culpa, I've belatedly looked at a cut-away. Shouldn't have relied on memory, 37 years on. You, ABUKABOY and barit1 didn't need any help from me, you were spot on.
I think barit1 is right, a slower advance of the throttles avoids the momentary drop in rpm as the prop comes out of ground-fine into governing mode. On the Herald, the latter was announced by the pitch lever moving forward (provided the controls had been unlocked), and the green ground-fine-pitch lights going out.
For the landing, the Viscount selects its own ground-fine, apparently, by the weight switches. On the Herald, the crew had to remember it. Don't know about the F27.
411A is right that there is a change of note if/when water-meth cuts in. The note gets heavier (more thrust from coarser pitch at the same maximum rpm?) and, on the Herald, heard externally, there was often a strange, slapping sound.
Quote from dixi188:
Also the -520 series Dart, on Viscounts and F27s, W/M restores power at hight ambient temps so would not be used in the UK very often except on hot summer days, where as the -532 on the Dart Herald boosted power at all temps. so was used more often.
[Unquote]
That would explain why on the Herald (only 2 engines, if my memory hasn't failed again)
we often used it, together with zero flap, to satisfy the Performance-A "WAT" limit (to improve the second-segment climb angle in the engine-out case); even with fairly modest summer temperatures.[Rather off-topic, "Whispering Giant" was a marketing slogan for the (Proteus) Britannia. Certainly quiet inside, from my (pax) experience. But, as you say, there was a deep rumble in the air, heard externally. The Bristol Proteus was 2-spool (I checked this time!), with axial-flow compressors on the N2, and 2 turbines per spool; but the air flowed forwards while being compressed, then reversing direction into the combustion chambers. This enabled the engine to be shorter (but caused the serious African icing problems early on). When taxying, at idle revs, it made an almost inaudible whisper (observed from the perimeter fence), occasionally interrupted by a "whoosh" - maybe as the props went into superfine pitch when the throttles were completely closed?
So unlike the deafening high-pitched song of the Dart... I would love to have a recording of the Proteus, but doubt finding one on a pop album.]
Join Date: Jan 2006
Location: AUCKLAND
Posts: 97
Likes: 0
Received 0 Likes
on
0 Posts
Back in 1979 as young Kiwi doing my OE I was working at LBA and got to ride in BMA Viscounts a number of times going down to London....I remember one day seeing the 10am flight from LHR arrive on a very wet and grey Leeds days....from my office window this one seemed to be going far too fast landing RWY 32....it was!!! took to the grass rather than go on to the busy Leeds-Harrogate road....as soon as it went onto the grass(it had been raining for a few days) the L/H gear collapsed and props on that side dug into the soft ground flinging grass & mud into the air...it swing about 45 degrees facing the runway and all pax and crew left the Aircraft without a scratch...and a few months later another went off the end of RWY 14 (this was before the runway was extended) I dont think Viscounts liked wet runways....One of New Zealand NAC (National Airways Corp) Viscount went off the runway at Wellington....can't remember what caused that one....but the sound of a Dart is something else.
Further to the 'Flight Fine' & 'Ground Fine' pitch discussion:
The -800 series VC8 had switches to motor the props to Flight Fine. Under normal circumstances there was no requirement to use these. Some misguided souls considered placing them to FF an amusing way to commence taxi. RPM would drop and TGT rise. Not a great idea.
Otherwise the system was automatic - can't remember whether it was controlled by weight-on switches.
The lever system with which the Argosy was fitted between #2 & #3 throttles clicked forward as the throttles were opened and armed the Flight Fine Pitch Stop and, on landing, had to be physically pulled back to remove the FFPS and permit the props to reduce to Ground Fine Pitch.
There was an embarassing incident caused by a take-off misunderstanding:
Power goes on and FFP lever clicks forward.
Crewmember asks if take off clearance has been received.
Throttles closed and clearance confirmed as having been received.
You can see where we're going now, can't you?
Throttles re-opened with props in FF and a couple of donks overtemped - soddit!
There but for the grace etc etc.
The -800 series VC8 had switches to motor the props to Flight Fine. Under normal circumstances there was no requirement to use these. Some misguided souls considered placing them to FF an amusing way to commence taxi. RPM would drop and TGT rise. Not a great idea.
Otherwise the system was automatic - can't remember whether it was controlled by weight-on switches.
The lever system with which the Argosy was fitted between #2 & #3 throttles clicked forward as the throttles were opened and armed the Flight Fine Pitch Stop and, on landing, had to be physically pulled back to remove the FFPS and permit the props to reduce to Ground Fine Pitch.
There was an embarassing incident caused by a take-off misunderstanding:
Power goes on and FFP lever clicks forward.
Crewmember asks if take off clearance has been received.
Throttles closed and clearance confirmed as having been received.
You can see where we're going now, can't you?
Throttles re-opened with props in FF and a couple of donks overtemped - soddit!
There but for the grace etc etc.
Join Date: May 2002
Location: Chilterns/Blighty
Posts: 478
Likes: 0
Received 0 Likes
on
0 Posts
Basil,
Have become all 'dewy-eyed' reading about Darts having left them behind 30+ years ago(Heralds and F27s).
Time we took tea at WW to catch up. Will PM in due course once I get a roster that will allow it.
Now back to thread!
FW
Have become all 'dewy-eyed' reading about Darts having left them behind 30+ years ago(Heralds and F27s).
Time we took tea at WW to catch up. Will PM in due course once I get a roster that will allow it.
Now back to thread!
FW
Thread Starter
Join Date: Feb 2006
Location: Bath
Age: 58
Posts: 42
Likes: 0
Received 0 Likes
on
0 Posts
Basil : quote : 'some misguided souls considered placing them to FF an amusing way to commence taxi. RPM would drop and TGT rise. Not a great idea.'
Is this the answer to my original question then? When would this switch have been triggered on take-off roll - just before pushing the levers forward or during the spool-up. If switched before governing mode but after start of spool-up, this would explain my 70's sound. Given it was not the best idea, perhaps operating procedures were changed to specifically prohibit it in the 80's ....
Chris Scott : as a Twin Wasp and Dart man, is there anything in my 'theory' (see previous post - more like an idle thought) that a piston driver would be in the habit (or be required by procedures) of advancing the throttles faster than a 'pure bred' turboprop driver?
Ref to my other thread - any ideas on the asymmetric Viscount tailplane?
Is this the answer to my original question then? When would this switch have been triggered on take-off roll - just before pushing the levers forward or during the spool-up. If switched before governing mode but after start of spool-up, this would explain my 70's sound. Given it was not the best idea, perhaps operating procedures were changed to specifically prohibit it in the 80's ....
Chris Scott : as a Twin Wasp and Dart man, is there anything in my 'theory' (see previous post - more like an idle thought) that a piston driver would be in the habit (or be required by procedures) of advancing the throttles faster than a 'pure bred' turboprop driver?
Ref to my other thread - any ideas on the asymmetric Viscount tailplane?
Join Date: Aug 2007
Posts: 647
Likes: 0
Received 0 Likes
on
0 Posts
Slightly off Topic (mods might roast me) - I do remember my 1st ever flight in june 66 - Brum to Dublin - Big windows and small 6 Yr old SLF - Reg G AOHH. A 702 Series ? . - Viscount and there was a momentarly drop in Prop RPM as the Throttles were opened for T/O on 33. seated RHS 4 rows from the back - Did those great Big Passenger windows mean that the VC8 Cruse Alt was restricted to conserve fatigue life in the VC8 ? .
FW
jh5speed,
IIRC the switches were a backup in case the flight fine pitch stop failed so would only (except by aforementioned, dare I say, mavericks) be placed to FFP in the event of a malfunction
CATIII-NDB,
Yes, the windows were amazing.
Last flew G-AOHH Glasgow, Machrihanish, Islay on 11 Sep 1975. Coincidentally, the model on my bookshelf is HH.
G-AOHH VISCOUNT 802
MTOW: 29257kg Year Built: 1957
C of A Expiry: 21/08/1977
IIRC, in the 70s, the cabin alt was increased slightly above the design figure to reduce pressure diff in deference to the age of the aircraft.
It's about 30 years since I flew them and defer to those who may add to the above.
jh5speed,
IIRC the switches were a backup in case the flight fine pitch stop failed so would only (except by aforementioned, dare I say, mavericks) be placed to FFP in the event of a malfunction
CATIII-NDB,
Yes, the windows were amazing.
Last flew G-AOHH Glasgow, Machrihanish, Islay on 11 Sep 1975. Coincidentally, the model on my bookshelf is HH.
G-AOHH VISCOUNT 802
MTOW: 29257kg Year Built: 1957
C of A Expiry: 21/08/1977
IIRC, in the 70s, the cabin alt was increased slightly above the design figure to reduce pressure diff in deference to the age of the aircraft.
It's about 30 years since I flew them and defer to those who may add to the above.
TLA Rates, Radials vs Dart
Quote from jh5speed:
Chris Scott :as a Twin Wasp and Dart man, is there anything in my 'theory' (see previous post - more like an idle thought) that a piston driver would be in the habit (or be required by procedures) of advancing the throttles faster than a 'pure bred' turboprop driver?
[Unquote]
I would say probably not. Only did 450 hrs on the Dak, 40 years ago, and no other big pistons, so am no expert...
But I think anyone throwing the throttles open or shut on a radial would be asking for an unpleasant surprise. [This is particularly true if you are doing a rolling T/O, as we generally did ('cos lots of runway).] The big beasts are not that reliable, and need to be nursed.
The only advantage of the Twin Wasp vs. the Dart is that you have selected the pitch levers on each engine to full fine (but still governed, I think) before T/O. The Dart, on the other hand, does not have a propeller pitch lever as such. [The one we've been talking about on this thread - on the Herald and Argosy - is purely to enable the crew to select GROUND-fine pitch for ground manoeuvering.]
So, on the Dart, the pilot has no control over engine rpm at a particular throttle setting - some widget decides it for him. But on most piston engines with CSU-governed props, of course, you can vary the engine rpm (by moving the pitch lever) without moving the throttle.
[The asymmetric Viscount tailplane is news to me, but you are right: the Dart props were not handed, as far as I can remember.
dxi188?]
Chris Scott :as a Twin Wasp and Dart man, is there anything in my 'theory' (see previous post - more like an idle thought) that a piston driver would be in the habit (or be required by procedures) of advancing the throttles faster than a 'pure bred' turboprop driver?
[Unquote]
I would say probably not. Only did 450 hrs on the Dak, 40 years ago, and no other big pistons, so am no expert...
But I think anyone throwing the throttles open or shut on a radial would be asking for an unpleasant surprise. [This is particularly true if you are doing a rolling T/O, as we generally did ('cos lots of runway).] The big beasts are not that reliable, and need to be nursed.
The only advantage of the Twin Wasp vs. the Dart is that you have selected the pitch levers on each engine to full fine (but still governed, I think) before T/O. The Dart, on the other hand, does not have a propeller pitch lever as such. [The one we've been talking about on this thread - on the Herald and Argosy - is purely to enable the crew to select GROUND-fine pitch for ground manoeuvering.]
So, on the Dart, the pilot has no control over engine rpm at a particular throttle setting - some widget decides it for him. But on most piston engines with CSU-governed props, of course, you can vary the engine rpm (by moving the pitch lever) without moving the throttle.
[The asymmetric Viscount tailplane is news to me, but you are right: the Dart props were not handed, as far as I can remember.
dxi188?]
Power Levers and Fuel-Trim switches
Quote from yours truly, above:
So, on the Dart, the pilot has no control over engine rpm at a particular throttle setting - some widget decides it for him.
[Unquote]
Must admit to having misgivings about the above, while out for dinner.
1) "Throttle" is the wrong term; the pilot controls each engine with what is called a Power Lever.
2) I got the cart before the horse. Once the engine power is increased sufficiently for T/O, and the propeller comes out of ground-fine pitch, the Power Lever is effectively an engine rpm selector. The "widget" (CASC unit?) has to decide the fuel flow appropriate for the rpm chosen. That delivered, the exact rpm is achieved by the blade-pitch adjustment of the Rotol propeller constant-speed unit. [The achieved engine torque is monitored, and if it falls below a certain low figure at take-off power, the propeller will auto-feather.]
3) The fuel-flow schedule, at a given rpm, can be modified by the crew. This is achieved by the fuel-trim switches, on a scale of 0 - 100. A fuel-trim figure for take-off is calculated and set in advance, according to the conditions. After take-off and during clean-up, the fuel trim is reduced (I think). At top of descent, it is reduced to zero, and descent rpm is set with the power lever. On the initial approach, fuel trim is increased, perhaps to 50. On finals, probably with the landing gear, it is increased to max (100), in preparation for a possible go-around.
If you get it right, the power levers (and, therefore, engine rpm) can remain untouched from TOD until the flare.
Does this sound familiar to you ex-Dart pilots? dxi188, does any of the above make sense?
So, on the Dart, the pilot has no control over engine rpm at a particular throttle setting - some widget decides it for him.
[Unquote]
Must admit to having misgivings about the above, while out for dinner.
1) "Throttle" is the wrong term; the pilot controls each engine with what is called a Power Lever.
2) I got the cart before the horse. Once the engine power is increased sufficiently for T/O, and the propeller comes out of ground-fine pitch, the Power Lever is effectively an engine rpm selector. The "widget" (CASC unit?) has to decide the fuel flow appropriate for the rpm chosen. That delivered, the exact rpm is achieved by the blade-pitch adjustment of the Rotol propeller constant-speed unit. [The achieved engine torque is monitored, and if it falls below a certain low figure at take-off power, the propeller will auto-feather.]
3) The fuel-flow schedule, at a given rpm, can be modified by the crew. This is achieved by the fuel-trim switches, on a scale of 0 - 100. A fuel-trim figure for take-off is calculated and set in advance, according to the conditions. After take-off and during clean-up, the fuel trim is reduced (I think). At top of descent, it is reduced to zero, and descent rpm is set with the power lever. On the initial approach, fuel trim is increased, perhaps to 50. On finals, probably with the landing gear, it is increased to max (100), in preparation for a possible go-around.
If you get it right, the power levers (and, therefore, engine rpm) can remain untouched from TOD until the flare.
Does this sound familiar to you ex-Dart pilots? dxi188, does any of the above make sense?
Join Date: Mar 2000
Location: Arizona USA
Posts: 8,571
Likes: 0
Received 0 Likes
on
0 Posts
So, on the Dart, the pilot has no control over engine rpm at a particular throttle setting - some widget decides it for him.
Prop and throttle (control) are combined, and together with the fuel trimmer, determine engine power output.
A completely reliable system, that operates superbly.
Join Date: Aug 2003
Location: Sale, Australia
Age: 80
Posts: 3,832
Likes: 0
Received 0 Likes
on
0 Posts
Could some one please explain the purpose of the fuel trim on the Dart. Chris Scott has touched upon its use but why the trim to modulate fuel flow? FCU of the day not up to doing the job properly or accurately enough?
Yes Chris it makes sense.
Throttle is directly connected to the PCU (prop control unit) and via a clever linkage to the FCU (fuel control unit). The electrical Fuel Trim Actuator is also connected via this linkage to the FCU.
This allows the RPM to be selected by the throttle and the power to be varied by the fuel trimmer. (+ or - 40 % I think)
I'd have to rummage in the loft to find a full description.
Typical power settings were as follows:-
Take off - 15000rpm max torque/ 100% trim
Climb - 14200rpm 760 degrees TGT set by trimmers (maybe 80% trim)
Cruise - 14200rpm 720 degrees TGT set by trimmers (maybe 60% trim)
These numbers are from memory and apply to the H.P. Herald.
Hope this helps.
P.S. CASC is "Combined Acceleration and Speed Control" as used on RR Spey not Dart.
Throttle is directly connected to the PCU (prop control unit) and via a clever linkage to the FCU (fuel control unit). The electrical Fuel Trim Actuator is also connected via this linkage to the FCU.
This allows the RPM to be selected by the throttle and the power to be varied by the fuel trimmer. (+ or - 40 % I think)
I'd have to rummage in the loft to find a full description.
Typical power settings were as follows:-
Take off - 15000rpm max torque/ 100% trim
Climb - 14200rpm 760 degrees TGT set by trimmers (maybe 80% trim)
Cruise - 14200rpm 720 degrees TGT set by trimmers (maybe 60% trim)
These numbers are from memory and apply to the H.P. Herald.
Hope this helps.
P.S. CASC is "Combined Acceleration and Speed Control" as used on RR Spey not Dart.
BA,
I've forgotten if we had fuel trimmers on the Viscount but we certainly had them on the Argosy.
Low level SOP was to keep them set to datum for the prevailing OAT.
People would sometimes use them to control speed which meant that, if set to less than datum, full power was not available when the throttles were opened - as a crew discovered when trying to climb over a hill following a slight nav error in the Far East.
Just made it but rumour was they picked up a bit of foliage on the way
Again TBFTGOGGI
dixi188,
Mil Arg was 14,500/785 for T/O
WTF do I remember that? Must be creeping senility
I've forgotten if we had fuel trimmers on the Viscount but we certainly had them on the Argosy.
Low level SOP was to keep them set to datum for the prevailing OAT.
People would sometimes use them to control speed which meant that, if set to less than datum, full power was not available when the throttles were opened - as a crew discovered when trying to climb over a hill following a slight nav error in the Far East.
Just made it but rumour was they picked up a bit of foliage on the way
Again TBFTGOGGI
dixi188,
Mil Arg was 14,500/785 for T/O
WTF do I remember that? Must be creeping senility
Join Date: Mar 2000
Location: Arizona USA
Posts: 8,571
Likes: 0
Received 0 Likes
on
0 Posts
IF the fuel trimmers are set improperly, at full throttle, the engines could well overtemp...the fuel trimmers are set according to the airfield pressure altitude and prevailing surface temperature, as reported by ATC, for takeoff and landing.
So, in essence, the Dart provided single lever engine power control, aided by manual fuel trim...FADEC not then being available.
So, in essence, the Dart provided single lever engine power control, aided by manual fuel trim...FADEC not then being available.
